Monday, October 19, 2015

What constitutes "nice" weather? That is a question that I have spent a lot of time thinking about over the years. Unfortunately there is no satisfactory answer. One person's nice day is reason enough for someone else to complain. We hear on the television newscast that the weather will be pleasant, or nice, or unsettled, or some other descriptive term that conforms to the expectation of what is most desirable.

Without conducting a survey, I am left to arbitrarily decide what constitutes a pleasant day. Based on my own life experiences, I will subjectively say that 70°F is an ideal temperature for a plurality of individuals. I know, I know, there will be a lot of complaints about this. Earlier this year I wrote blog posts about Picture Perfect Weather Days and Dreary Weather. Please feel free to check out those posts for more thoughts on what constitute ideal, or crappy, weather.

Road Trip(s)

Imagine that you really like days where the high temperature is 70°F and you wanted to plan a road trip where the temperature always hovers around 70°F. Well, I have done the planning for you. Since you cannot know what the temperature will be more than a week in advance (the current limit of decent forecasts), you have to go with Plan B; that is, look in a climate almanac. The go-to climate almanac for United States data is published by the National Center for Environmental Information (NCEI). The daily normal high temperature is computed for over 8,000 stations all across the United States. In addition, Environment Canada publishes monthly normals for stations across the great north. (Note: "normal" is a technical term that refers to a smoothed average for a 30-year period. It is close to an arithmetic average but not exactly the same.)

North AmericaWhat if have a passport and decided to expand our travels to include Canada and Alaska (no passport required). The portion of the trip that begins in deep south Texas on January 1 all the way through the first part of June remains the same as before.

Figure 1. Proposed route across North America where the normal high temperature is always at, or very near 70°F.

On January 1, the normal high temperature in deep south Texas is 70°F. This is a good place to start the trip.

By February 1, our road trip has only progresses as far as Laredo, Texas. The next two months will be spent on Interstate 35.

On March 1, the slow trek is only pulling up to the south part of San Antonio, Texas.

Throughout the month of March, the higher sun angle warms things up and the trip has taken us to the Texas/Oklahoma border by April 1st.

Now that April is upon us, we start heading east and northeast until we near the nation's Capitol by the end of the month.

During May, we go through Pittsburgh and Chicago and end the month in extreme northern Wisconsin on June 1st.

A tour of the northern states is in order to during June. In fact, we go more miles in June than any other month. Starting around the second week of June, our route makes a right turn on to the Alcan Highway. We travel through Calgary and go past Whitehorse on our way to and through Alaska's two largest cities. The month ends near Whitehorse (again) heading southeast.

In July, we turn on Highway 37 and head south toward Bellingham, Washington, and finish the month in Portland, Oregon.

The August portion of the trip is a scenic section along Highway 101 through Oregon.

In September, we loop back onto Interstate 5 before turning due east in Eugene, Oregon. We continue through to the Snake River Plain in Idaho before turning south through South Dakota and into Nebraska.

In October, the trip makes its way to near Kansas City, Kansas, Tulsa, Oklahoma, and ends at the Texas/New Mexico state line.

The November portion of the trip follows Interstate 10 from Las Cruces, New Mexico, to Arizona, where it then heads north along Interstate 40, before departing for Las Vegas, Nevada, and ending near Los Angeles, California.

December is another leisurely month that only takes us from Los Angeles to San Diego, California.

This version of the route traveled through parts of 31 states and 3 Canadian provinces. All-in-all we put over 13,000 miles on our vehicle.

Figure 2. Total mileage and average miles per day for route shown in Figure 1.

Contiguous U.S.
The Contiguous U.S. version of the road trip is identical to the North American version up through the second week of June. Instead of heading into Canada, it hugs the Canadian border. Then ...

Near Glacier National Park, the route turns south along Highway 93 and passes Yellowstone National Park and Grand Teton National Park. The route then turns southeast and travels through central Wyoming before settling near Denver, Colorado.

The next two months are spent leisurely traversing the Colorado high country ending up near Durango, Colorado.

Once we make it to September, our trip takes us back through Yellowstone National Park and southern Montana, before turning south through South Dakota and into Nebraska.

The period of October through the end of the year are then identical to the North America route shown in Figure 1.

This version of the route traveled through parts of 30 states. All-in-all we put over 9,000 miles on our vehicle.

Figure 3. Proposed route across Contiguous U.S. where the normal high temperature is always at, or very near 70°F.

Daily Visualization

If you are really interested in 70°F temperatures, here is a YouTube video that I put together showing the stations with a high temperature within 1°F of 70°F for every day of the year.

Figure 4. YouTube video showing the progression of 70°F days throughout the year.

Addendum: 80°F Road Trip Map

Figure 5. Proposed route across Contiguous U.S. where the normal high temperature is always at, or very near 80°F.

Figure 6. YouTube video showing the progression of 80°F days throughout the year.

Saturday, October 3, 2015

Was the October 2000 cold snap in South Texas the most extreme temperature event in U.S. climate history when looking at high temperatures? Perhaps. Let's take a look. But first, some background.

NCDC Normals

There are certain times during the year, usually winter, where large temperature swings are not uncommon. In fact, portions of the Great Plains can expect 24-hour temperature changes of 50°F at least once per year. The variability in daily temperatures is measured statistically by calculating the standard deviation for a 30-year period and smoothing it over a 15-day moving filter. For the 1981-2010 time period, the National Climate Data Center (NCDC) has published daily normal temperatures and daily standard deviations for over 6,000 stations nationwide. These values are used to assess how anomalous daily temperature reading are in comparison to the current time period. Unfortunately, this data does not extend back to earlier 30-year normal periods. The NCDC changed their computational methodology for the 1981-2010 climate normal period and reconstructing previous epochs is nearly impossible.

Comparing a reading from January 1, 1950 (for example), to the 1981-2010 climate normal period is not entirely appropriate. The 30-year baseline was developed by the World Meteorological Organization to account for slow changes in the climate of a location. In the case of the 1981-2010 climate normals, they are the comparison standard for data collected between 2011 and 2020. That being said, we can still look at the January 1, 1950, data and state how far above or below it was in comparison to the 1981-2010 normal with the understanding that it may be somewhat of an apples to oranges comparison.

September/October 2000

Early September produced all-time record high temperatures (since broken) for areas between Central Texas and Mississippi. This blog's author spent an entire week outside in Northeast Louisiana in 106°F+ temperatures conducting fieldwork during the record streak and can attest to the severity of the heat. By mid-September the temperatures had returned to seasonal normals and some record lows were even set in the Deep South. However, this was just a taste of things to come.

As early as October 1st, forecasters were calling for a major buckle in the jest stream to drive cold air down through the center of the U.S. Figure 1 shows a 48-hour forecast map issued on October 1, 2000, The leading edge of the cold front is forecasted to drive through North Dakota and Minnesota at that time. The text portion of the forecast calls, underlined in the figure, calls for the cold air to make it all the way to the Deep South.

By October 5th, the Arctic cold had already swept from Montana through the Front Range and began to enter the Texas Panhandle. Temperatures were 10°F-15°F below normal north of Lubbock. On the 6th, the front pushed through Central Texas and on the 7th it made it down to deep South Texas (see Figure 2). Not only were temperatures substantially below normal, but they were historically below normal.

Figure 2. Surface weather map from October 7, 2000.

The chart depicted in Figure 3 shows the hourly temperature observations at Houston's Bush Intercontinental Airport between October 6th and October 11th, 2000. The cold front arrived at the airport during the evening of October 6th and temperatures dropped all day through October 7th. There was a 37-hour period where the temperature did not exceed 50°F.

Figure 3. Hourly temperatures at George Bush Intercontinental Airport from October 6-11, 2000. The average hourly temperature is shown as a dashed line. Note: year 2000 hourly observations were not included in the average hourly calculations.

Nationwide Analysis

When querying the Global Historical Climate Network (GHCN) database for high temperature anomalies as compared to current (1981-2010) normals, a single event stands out. Figure 4 shows the sixteen greatest daily high temperature departures from current normals in the GHCN database at 1st-Order stations. Cooperative stations were excluded from the analysis due to time of day observations issues.

Figure 4. Greatest high temperature departures from 1981-2010 climate normals for all airport stations in the GHCN v.3 database. All entries have passed an initial quality control check. Hourly observations from Lihue in October 1965 appear to validate their cool October day in 1965.

Twelve of the fifteen most anomalous events were recorded in South Texas between October 8 and October 10, 2000. The Port Isabel high temperature of 51°F on 10/9/2000 is one of only two days in the GHCN database that if the observation were to occur today would be more than eight standard deviations below normal. If you are not familiar with what a standard deviation is, here is a brief description. If temperature data are normally distributed, which they generally are, we expect 68% of daily observation to fall within 1 standard deviation of the daily normal; 95% to fall within 2 standard deviations of the daily normal; and 99.7% to fall within 3 standard deviations of the daily normal. Considering that approximately 18 million daily climate records exist in the GHCN database, and only two are more than 8 standard deviations from the daily normal, this was truly a historic event.

The next two maps show the maximum high temperature departure from the 1981-2010 normal (Figure 5) and the maximum standard deviation from the 1981-2010 normal (Figure 6).

Figure 5. Largest daily temperature departure from normal for entire U.S. between October 8-10, 2000.

Figure 6. Largest daily standard deviation departure from normal for entire U.S. between October 8-10, 2000.

Texas Maps

While large portions of the U.S. were significantly below normal, the core of anomalously low temperatures was south of Interstate 10 in Texas. The following three figures show just how unusual the high temperatures were in Texas. Each figure is color shaded and primary stations are marked and labelled. The first figure (Figure 7) shows the coldest high temperature during the three-day period. The next figure (Figure 8) shows the greatest high temperature departure from normal during those same three days and finally, Figure 9 shows the number of standard deviation below normal for the coldest day during that period.

Figure 7. Lowest high temperature for Texas and vicinity between October 8-10, 2000.

Figure 8. Largest daily temperature departure from normal for Texas and vicinity between October 8-10, 2000.

Figure 9. Largest daily standard deviation departure from normal for Texas and vicinity between October 8-10, 2000.

Why so Cold?

So why was it so anomalously cold in Texas? There are several answers. First, the airmass was especially cold. Figure 10 shows the upper air conditions from the Corpus Christi, TX, RAOB sounding on 10/9/00 0Z. The temperature at 850 mb is only 3.7°C. This is, by far, the lowest for so early in the season.

Figure 10. Upper air sounding for Corpus Christi, TX, at 0Z on October 9, 2000.

Just as importantly, clouds and light rain were present. If the skies were clear and the 850 mb temp were low, that would drive record low minimum temperatures. However, this post is about record low maximum temperature. An overrunning situation took shape with moist air lifting over the shallow cold airmass (isentropic upglide). The cloud cover (see Figure 11) prevented solar energy from warming the surface, and the precipitation falling through cold airmass acted to suppress temperatures due to latent heat processes.

Figure 11. GOES-8 infrared satellite image on October 8, 2000.

Conclusion
If you live in Montana, a temperature of 35°F below normal is nothing to write home about. But that same discrepancy in Texas, in October, is one for the record books. If you live in South Texas, go outside on October 8-10 of this year and imagine what temperatures in the 40s feel like.